The present invention relates to adhesive proteins which have been isolated from mature plants such as algae. The isolated proteins may be but are not necessarily, those utilized by the plants for adhesion to substrates. The invention also relates to a process for isolating such proteins.
Algae are known as commercial sources of food and polysaccharides such as agar, alginates, carrageenan and furcellaran. Many such polysaccharides possess adhesive properties and are therefore used as gums and adhesives, and in wound dressings.
Apart from the existing industrial practice of extracting polysaccharide gums from algae, interest has been shown in algae and other plants as a source for pharmaceuticals or compounds which are otherwise bioactive, as well as in the means and adhesive substances by which algae and other plants, including certain bacteria and fungi, adhere to substrates in nature.
Walker, G. in Chapter 5 of "Synthetic Adhesives and Sealants", ed. Wake, W. C., publ. Society of Chemical Industry, 1987, page 117, mentions that in macroalgae the adhesives used by spores and the rhizoid disc adhesives are polysaccharide-protein complexes, while filamentous rhizoid adhesive is a complex polysaccharide only. This reference also mentions (page 117) that in Laminaria digitata, mucilage with a high phenolic content --apparently contained in tannins--plays a role in cementing the rhizoid to the substrate, the presence of phenolic compounds possibly leading to hardening of the cement (c.f. barnacle and mussel adhesion, and see below) and renders the cement/substrate bond immune to growth of microorganisms; and at page 118, in connection with microalgae, it is reporter that the adhesive of A. veneta is composed of a complex polysaccharide and that no protein or lipid was detected.
It is known that proteins which appear to serve chiefly as waterproof adhesives and varnishes in nature, are widely distributed throughout the animal kingdom. In particular, the chemical and physical stability of such materials is believed to be imparted thereto by quinone tanning of polyphenolic proteins, implying that their desirable waterproof/adhesive properties is due to catechol oxidase-catalyzed oxidation of a structural unit in the protein derived from 3,4-dihydroxyphenyl-L-alanine (DOPA) to a corresponding 2-(3,4-dioxo-3,4-dihydrobenzyl)glycine unit, which is then capable of undergoing numerous reactions, such that the potential adhesives and varnishes would be cured. (See, e.g., "The Phylogeny and Chemical Diversity of Quinone-Tanned Glues and Varnishes", Waite, J. H., Comp. Biochem. Physiol., 97B(1): 19-29 (1990)). In this connection, it has been noted, e.g., in an article by Saez, C. et al, Comp. Biochem, Physiol., 98B(4): 569-72 (1991) that polyphenolic adhesive proteins extracted from three species of marine mussels contain 7.9-17.4% DOPA; two of the three species contain 13.1-17.4% DOPA and 17.8-18.8% of the basic aminoacid lysine.
However, the general differences between the animal and plant kingdoms are fairly well known. Plant cell walls are usually polysaccharidic in nature and plants are usually non-motile, feed by absorption and/or photosynthesis, and can synthesize aminoacids, and thus proteins. Animal cell membranes are by contrast not polysaccharidic, and animals feed by digestion and do not generally photosynthesize.
Wagner, V. T., et al, in P.N.A.S. (USA) 89: 3644-3648 (1992), described a vitronectin-like glycoprotein (Vn-F) isolated from zygotes and two-celled embryos, which were in turn propagated from material obtained from reproductive fronds of Fucus distichus. Vn-F is said to be exclusively localized in the cell wall of the rhizoid tip, at the site of contact between the Fucus embryo and its substrate. This article appears to imply a parallelism between the structure and function of Vn-F in Fucus embryos, and that of vitronectin (Vn) in mammals which together with other extracellular matrix (ECM) proteins are linked structurally and functionally through the plasma membrane to the cytoskeleton via a family of transmembrane proteins. This article also mentions briefly that a Vn-like protein was isolated from the acellular slime mold Physarum, but this mention does not refer to adhesion. The authors indicate that their data suggest evolutionary conservation of structure and function of Vn between brown algae and mammals. However, this suggestion has not found ready acceptance in the field, because, inter alia, similarity of functions between "Vn-like" proteins and animal vitronectins has not been confirmed. For example, Zhu, J.-K. et al in The Plant Cell. 6: 393-404 (1994) posit the possibility that plant Vn-like proteins have little or no primary sequence identity with animal vitronectin. It may also be noted in passing that although vitronectins and vitronectin-like proteins are referred to as "adhesion proteins", the adhesion in question so far as proteins of animal origin are concerned, appears to be principally adhesion within an organism, whereas such proteins of plant origin may exhibit additionally adhesion to substrates, e.g. in the case of algae.